System and method for multiple input multiple output wireless transmission

ABSTRACT

Systems and methods for MIMO wireless transmission are provided. At the transmitter, a plurality of encoded packets are modulated, and the symbols are divided between several transmit antennas. The transmitter spreads some of the symbols of each packet using a respective subset of an available Walsh code space. A given transmit antenna then transmits part of each packet spread by the respective subset. In some implementations, this achieves the benefits of the PARC system, and at the same time achieves part of the benefits of the STTD system. Advantageously, only a single reverse link control channel is required if adaptive coding and modulation is to be performed.

FIELD OF THE INVENTION

The invention relates to systems and methods for multiple input multipleoutput wireless transmission.

BACKGROUND OF THE INVENTION

To increase cellular system capacity, one approach that shows promisefor substantial capacity enhancement is multiple input and multipleoutput (MIMO) transmission and reception based on multiple transmit andmultiple receive antennas. This has been suggested for use on forwardlink channels such as the forward-packet-data channel (F-PDCH) in1xEV-DV [1,2]. To further increase system capacity, Lucent proposed a2×2 MIMO architecture based on the per-antenna-rate-control (PARC)principle [3]. This scheme has advantages in terms of average high datarates achieved compared to single stream transmission schemes such asthose used with minimum-mean square error (MMSE) based dual receivediversity mobile station (MS), phase sweeping transmit diversity (PSTD)and space-time transmit diversity (STTD/STS). One of the main drawbackswith PARC, however, is the high residual frame error rate (FER) comparedwith the simple dual-receive diversity MS. Thus, it requires a largenumber of retransmissions to complete a data transmission, particularlyin high velocity environments.

STTD is an open loop technique in which the symbols are modulated usingthe technique of space-time block coding described in [4]. The STTDtransmitter and STTD receiver are illustrated in FIG. 1. FIG. 1 showstwo symbols S₁, S₂ entering an STTD encoder 10. This produces at itsoutput a STTD signal to be transmitted over to transmit antennas 12,14.During the first transmit period, w₁S₁ is transmitted on the firstantenna and −w₂S₂* is transmitted on the second antenna 14. In thefollowing transmit period, w₁S₂ is transmitted on the first antenna 12,and w₂S₁* is transmitted on the second antenna 14. It can be seen thatthe same information is transmitted twice, both during different timesand on different antennas. This is transmitted to receive antennas 16and 18. There is a respective channel between each of the transmitantennas 12,14 and each of the receive antennas 16,18. The four channelsare shown as h₁₁, h₁₂, h₂₁, and h₂₂. The signals received by the tworeceive antennas 16,18 are passed to an STTD decoder 20 which recoversreceived versions S₂ ⁻,S₁ ⁻ of the transmitted symbols S₁,S₂. In FIG. 1,the expression w₁S₁ represents a Walsh space multiplied by a transmitpacket. More particularly, if a Walsh space is available consisting of MWalsh codes is available, this Walsh space is divided among the twoantennas, and the contents of the packet divided into segments fortransmission using each Walsh code. Thus, in a 16 Walsh code Walshspace, 8 Walsh codes would be transmitted on each transmit antenna, andthe transmit packet would be divided into 8 segments with one segmentbeing transmitted using each Walsh code. Each symbol might be an MPSKsymbol for example or any other suitable modulation symbol. However, forspace-time encoding it is necessary that the symbols lend themselves tocomplex conjugation.

The STTD scheme is particularly simple. It implements the space-timeblock code (2×2 code matrices). The orthogonality property of the codematrices allows the symbols from the two transmit antennas to beseparated at the receiver. Thus, it may achieve a significant codinggain on space and time as opposed to other diversity schemes such asorthogonal transmit diversity (OTD).

The PARC MIMO system is illustrated in FIG. 2. In this example, the datasequence can be seen to be demultiplexed into two separate streams30,32. Each stream is assigned an independent modulation/coding/rate.Each of the two streams is used in BLAST encoder 31 to generate aseparate Walsh code spread signal that is transmitted from only one ofthe antennas. In this case, the entire available Walsh space is assignedon both antennas, and as such there is no Walsh code splitting. This isillustrated by showing each symbol being multiplied by the combinationof w₁+w₂. This means that the packet is divided into a number ofsegments equal to the number of Walsh codes and an equal portion of thepacket is transmitted using each Walsh code. For example, if there are16 Walsh codes in the available Walsh space, the packet would be dividedinto 16 portions and each portion transmitted using a respective Walshcode. It can be seen that there is no space diversity at the transmitteror time diversity at the transmitter. However, at the receiver there isspace diversity since the signal is received at two different antennas.

This design is capable of increasing the system capacity by a factor oftwo as opposed to 1xEV-DV system.

It should be noted that the transmit encoder packets may belong toeither the same user or different users, resulting in differentrequirements for control signaling. In case of the same user served bytwo transmit antennas, this requires all encoder packets to occupy thesame number of transmission slots so that the transmission for thepackets can be always started at the same time. This requires twofeedback signal for transmit packet length but requires two feedbacksignals for packet rate control due to the closed loop MIMO scheme. Incase of the different users served by different transmit antennas, thisdoes not require encoder packets to occupy the same number oftransmission slots, but requires twice the feedback information fortransmit packet length.

Even when the encoder packets belong to the same user, this system stillrequires two feedback channels (packet data control channel, PDCCH).This is because the channel feed back signal includes the number ofpackets, modulation and code rate information (in this case, themodulation and code rate, in general, are not the same on differentantenna due to the difference reported CIR), and it is not possible toemploy only one channel to complete such information feedback.

SUMMARY OF THE INVENTION

Systems and methods for MIMO wireless transmission are provided. At thetransmitter, a plurality of encoded packets are modulated, and thesymbols are divided between several transmit antennas. The transmitterspreads some of the symbols of each packet using a respective subset ofan available Walsh code space. A given transmit antenna then transmitspart of each packet spread by the respective subset. In someimplementations, this achieves the benefits of the PARC system, and atthe same time achieves part of the benefits of the STTD system.Advantageously, only a single reverse link control channel is requiredif adaptive coding and modulation is to be performed.

According to one broad aspect, the invention provides a method oftransmitting a first set of modulation symbols of a first encoded packetcomprising: spreading a first subset of the first set of modulationsymbols using a first subset of an orthogonal code set to generate afirst spread signal, and transmitting the first spread signal on a firstantenna; spreading a second subset of the first set of modulationsymbols using a second subset of the orthogonal code set to generate asecond spread signal and transmitting the second spread signal on asecond antenna.

In some embodiments, the first subset of the orthogonal code set isdistinct from the second subset of the orthogonal code set.

In some embodiments, the method further comprises transmitting a secondset of modulation symbols of a second encoded packet by: spreading afirst subset of the second set of modulation symbols using the firstsubset of the orthogonal code set to further generate said second spreadsignal; spreading a second subset of the second set of modulationsymbols using the second subset of the orthogonal code set to furthergenerate said first spread signal.

In some embodiments, the orthogonal code set comprises a complete orincomplete set of Walsh codes.

In some embodiments, the method further comprises: performing a firstencoding of a first raw information stream to generate the first encodedpacket and performing a first modulation of the first encoded packet togenerate the first set of modulation symbols.

In some embodiments, the method further comprises: receiving channelquality information over a first reverse link control channel, thechannel quality information being generated by a receiver of the firstencoded packet; adaptively controlling the first encoding and the firstmodulation as a function of the first channel quality informationreceived over the reverse link control channel.

In some embodiments, the channel quality information comprises anaverage carrier to interference ratio experienced across multipleantennas at a receiver of the first encoded packet.

In some embodiments, the first encoded packet and the second encodedpacket belong to a single user, the method further comprising:transmitting a single forward link control channel identifying one ormore of number of slots, code rate and modulation type.

In some embodiments, the first encoded packet and the second encodedpacket each belong to a different user, the method further comprising:for each different user, transmitting a respective forward link controlchannel identifying one or more of number of slots, transmission rate,code rate/type and modulation type.

In some embodiments, the first encoded packet and the second encodedpacket are for a single user, the method further comprising: performingencoding of a raw information stream to generate the first encodedpacket and the second encoded packet and performing modulation of thefirst encoded packet and the second encoded packet to generate the firstset of modulation symbols and the second set of modulation symbols.

In some embodiments, the method further comprises: receiving channelquality information over a reverse link control channel from a receiverof the first encoded packet and the second encoded packet; adaptivelycontrolling the encoding and the modulation as a function of the channelquality information received over the reverse link control channel.

In some embodiments, the first encoded packet is for a first user andthe second encoded packet is for a second user, the method furthercomprising: performing a first encoding of first raw information streamto generate the first encoded packet and performing a first modulationof the first encoded packet to generate the first set of modulationsymbols; performing a second encoding of a second raw information streamto generate the second encoded packet and performing a second modulationof the second encoded packet to generate the second set of modulationsymbols.

In some embodiments, the method further comprises: receiving channelquality information over a first reverse link control channel from areceiver of the first encoded packet; receiving channel qualityinformation over a second reverse link control channel from a receiverof the second encoded packet; adaptively controlling the first encodingand the first modulation as a function of the channel qualityinformation received over the first reverse link control channel;adaptively controlling the second encoding and the second modulation asa function of the channel quality information received over the secondreverse link control channel.

In some embodiments, the first reverse quality control channel carrieschannel quality information comprising an average carrier tointerference ratio experienced across multiple antennas at the receiverof the first encoded packet; wherein the second reverse quality controlchannel carries channel quality information comprising an averagecarrier to interference ratio experienced across multiple antennas atthe receiver of the second encoded packet.

According to another broad aspect, the invention provides a methodcomprising: for each of a plurality M of transmit antennas where M>=2,generating a respective spread signal by: spreading a respective subsetof each of a plurality M sets of modulation symbols each associated witha respective encoded packet using a respective subset of an orthogonalcode set and transmitting the respective spread signal on the transmitantenna, the respective subsets used for the antenna beingnon-overlapping.

In some embodiments, the method further comprises: for each differentreceiver to which at least one of said M sets of modulation symbols isto be transmitted: a) receiving a respective channel quality informationfrom the receiver; b) performing adaptive modulation and encoding as afunction of the channel quality information; c) transmitting respectiveforward link control information to the receiver identifying theadaptive modulation and encoding performed.

In some embodiments, the respective subsets used on different transmitantennas for different subsets of the same set of modulation symbols arealso non-overlapping.

According to another broad aspect, the invention provides a method ofreceiving a signal from a transmitter comprising: receiving a signal ateach of a plurality of antennas, each signal being a combination ofsignals generated using a MAT (mix antenna transmission) transmitscheme; performing decoding of the signals to recover at least onepacket.

In some embodiments, the method further comprises: generating an averagesignal quality metric from the signals and transmitting this back to thetransmitter.

According to another broad aspect, the invention provides a transmitterfor transmitting a first set of modulation symbols of a first encodedpacket, the transmitter comprising: a first spreading function adaptedto spread a first subset of the first set of modulation symbols using afirst subset of an orthogonal code set to generate a first spreadsignal; a first antenna for transmitting the first spread signal; asecond spreading function adapted to spread a second subset of the firstset of modulation symbols using a second subset of the orthogonal codeset to generate a second spread signal; a second antenna fortransmitting the second spread signal.

In some embodiments, the first subset of the orthogonal code set isdistinct from the second subset of the orthogonal code set.

In some embodiments, the transmitter further adapts to transmit a secondset of modulation symbols of a second encoded packet wherein: the secondspreading function is further adapted to spread a first subset of thesecond set of modulation symbols using the first subset of theorthogonal code set to further generate said second spread signal; thefirst spreading function is further adapted to spread a second subset ofthe second set of modulation symbols using the first subset of theorthogonal code set to further generate said first spread signal.

In some embodiments, the orthogonal code set comprises a complete orincomplete set of Walsh codes.

In some embodiments, the transmitter further comprises: a first encoderadapted to perform a first encoding of a first raw information stream togenerate the first encoded packet; a first modulator adapted to performa first modulation of the first encoded packet to generate the first setof modulation symbols.

In some embodiments, the transmitter further comprises: an input forreceiving channel quality information over a first reverse link controlchannel, the channel quality information being generated by a receiverof the first encoded packet; an adaptive controller for adaptivelycontrolling the first encoding and the first modulation as a function ofthe first channel quality information received over the reverse linkcontrol channel.

In some embodiments, the first encoded packet and the second encodedpacket belong to a single user, the transmitter being further adapted totransmit a single forward link control channel identifying one or moreof number of slots, code rate and modulation type.

In some embodiments, the first encoded packet and the second encodedpacket each belong to a different user, the transmitter being furtheradapted to for each different user, transmit a respective forward linkcontrol channel identifying one or more of number of slots, transmissionrate, code rate/type and modulation type.

In some embodiments, the first encoded packet and the second encodedpacket are for a single user, the transmitter further comprising: anencoder for performing encoding of a raw information stream to generatethe first and second encoded packets; a modulator for performingmodulation of the first and second encoded packets to generate the firstand second sets of modulation symbols.

In some embodiments, the transmitter further comprises: an input forreceiving channel quality information over a reverse link controlchannel from a receiver of the first encoded packet and the secondencoded packet; an adaptive controller for adaptively controlling theencoding and the modulation as a function of the channel qualityinformation received over the reverse link control channel.

In some embodiments, the first encoded packet is for a first user andthe second encoded packet is for a second user, the transmitter furthercomprising: a first encoder for performing a first encoding of a firstraw information stream to generate the first encoded packet; a firstmodulator for performing first modulation of the first encoded packet togenerate the first set of modulation symbols; a second encoder forperforming a second encoding of a second raw information stream togenerate the second encoded packet; a second modulator for performing asecond modulation of the second encoded packet to generate the secondset of modulation symbols.

In some embodiments, the transmitter further comprises: a first inputfor receiving channel quality information over a first reverse linkcontrol channel from a receiver of the first encoded packet; a secondinput for receiving channel quality information over a second reverselink control channel from a receiver of the second encoded packet; afirst adaptive controller for adaptively controlling the first encodingand the first modulation as a function of the channel qualityinformation received over the first reverse link control channel; asecond adaptive controller for adaptively controlling the secondencoding and the second modulation as a function of the channel qualityinformation received over the second reverse link control channel.

According to another broad aspect, the invention provides a transmittercomprising: M transmit antennas; signal spreading functions which foreach of the plurality M of transmit antennas where M>=2, generaterespective spread signal by: spreading a respective subset of each of aplurality M sets of modulation symbols each associated with a respectiveencoded packet using a respective subset of an orthogonal code set andtransmitting the respective spread signal on the transmit antenna, therespective subsets used for the antenna being non-overlapping.

In some embodiments, the respective subsets used on different transmitantennas for different subsets of the same set of modulation symbols arealso non-overlapping.

According to another broad aspect, the invention provides a receivercomprising: a plurality of receive antennas each receiving a signalwhich is a combination of signals generated using a MAT (mix antennatransmission) transmit scheme; a decoder adapted to perform decoding ofthe signals to recover at least one of packet.

In some embodiments, the receiver further adapts to generate an averagesignal quality metric from the signals and transmit this back to thetransmitter.

According to another broad aspect, the invention provides a systemcomprising: a) a transmitter comprising: M transmit antennas; signalspreading functions which for each of the plurality M of transmitantennas where M>=2, generate respective spread signal by: spreading arespective subset of each of a plurality M sets of modulation symbolseach associated with a respective encoded packet using a respectivesubset of an orthogonal code set and transmitting the respective spreadsignal on the transmit antenna, the respective subsets used for theantenna being non-overlapping; b) at least two receivers each comprisingat least two receive antennas, each receiver being adapted to performdecoding and modulation of signals received from the transmitter torecover at least one respective packet.

In some embodiments, each receiver is further adapted to generate asignal quality information from signals received by the receiver and totransmit this back to the transmitter; wherein the transmitter furthercomprises: for each receiver, a channel quality input for receivingchannel quality information from the receiver; for each receiver, arespective encoder adapted to perform an encoding of a respective rawinformation stream to generate a respective encoded packet; for eachreceiver, a respective modulator adapted to perform a modulation of therespective encoded packet to generate the respective set of modulationsymbols; wherein the transmitter adaptively controls the encoding andthe modulation performed for each receiver as a function of the channelquality information received from the receiver.

In some embodiments, the transmitter is further adapted to, for eachdifferent user, transmit a respective forward link control channelidentifying one or more of number of slots, code rate and modulationtype.

According to another broad aspect, the invention provides a transmitterfor generating a plurality M of spread signals, where M>=2, thetransmitter comprising: M transmit antennas; for each transmit antenna,means for spreading a respective subset of each of a plurality M sets ofmodulation symbols each associated with a respective encoded packetusing a respective subset of an orthogonal code set; wherein therespective spread signal is transmit on the transmit antenna, with therespective subsets used for the antenna being non-overlapping.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the attached drawings in which:

FIG. 1 is a block diagram of a conventional STTD based transmit scheme;

FIG. 2 is a block diagram of a conventional PARC based transmissionscheme;

FIG. 3 is a block diagram of a MAT (mix antenna transmission) basedscheme provided by an embodiment of the invention;

FIG. 4A is a more detailed block diagram of a multi-user MAT basedscheme provided by an embodiment of the invention;

FIG. 4B is a more detailed block diagram of a single-user MAT basedscheme provided by an embodiment of the invention; and

FIGS. 5 through 9 provide simulation results.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a simplified example of a mix-antenna transmission (MAT)scheme provided by the invention which is based somewhat on theprinciples of PARC for MIMO systems. The motivation for this scheme isto increase the efficiency for both transmit antenna diversity and Walshcode reuse. The example is focusing on a two transmit antenna by tworeceive antenna embodiment. However, as will be described further belowthis is easily generalized to a generic MIMO application. Furthermore,while a single two receive antenna receiver is shown, multiple receiversof different users can receive the signals sent on the transmitantennas. Each user's signals would experience a different channel foreach transmit antenna+receive antenna permutation. At the transmitter, afirst input 60 carries a signal S₁ which represents a sequence ofmodulation symbols generated from an encoded packet. Similarly, input 62carries S₂, a sequence of modulation symbols generated from a secondencoded packet. The first sequence of encoded symbols S₁ is split intotwo parts α₁₁ and α₁₂. Similarly, the second set of modulation symbolsis split into two parts labelled α₂₁ and α₂₂. Block 64 represents Walshcode spreading that is applied for the signal to be transmitted on afirst transmit antenna 70, and block 66 represents Walsh code spreadingwhich is performed for the signal to be transmitted on the secondtransmit antenna 72. Half of the first set of modulation symbols, namelyα₁₁ and half of the second set of modulation symbols, namely α₂₁ areprocessed in Walsh code spreading block 64. Half of the Walsh code spaceis used to spread all while the other half of the Walsh code space isused to spread α₂₁. Similarly, in Walsh code spreading block 66, half ofthe Walsh code space is used to spread α₁₂ while the other half of theWalsh code space is used to spread α₂₂. The Walsh code space used tospread all in Walsh code spreading block 64 is different from the Walshcode space used to spread α₁₂, the other half of the same set ofmodulation symbols S₁. The two antennas transmit simultaneously, and thereceiver is shown also with two antennas 74,76. MMSE decoding isperformed to recover the transmitted symbol stream. At the receiver theencoder packet is recovered based on the received signals from twodifferent transmit antennas. This operation results in that the MATscheme not only preserves the benefits of the previous PARC scheme, butalso achieves the additional antenna diversity gain. More generally, anytype of decoding can be performed at the receiver at the discretion ofthe receiver designer. For example, a RAKE receiver design mightalternatively be employed.

A much more detailed example will now be described with reference toFIG. 4A. This is another example of a two antenna transmitter. In thisexample, it is assumed that there are two users to which data is to betransmitted. Each user has a two antenna receiver. Only one user'sequipment is shown. A first set of data, 100 consisting of bits {b₁,b₂ .. . } is input to user one encoder 104 which performs encoding with coderate R₁. This generates a first encoded packet 105 which is input touser one modulator 108 which performs modulation M₁. This might forexample be one of 16 QAM, 8PSK or QPSK. The output of the user onemodulator 108 is a first set of modulation symbols 109 consisting ofsymbols S₁, . . . , S_(N). Similarly, for a second data stream 102consisting of bits {d₁, d₂ . . . }, this is input to user two encoder106 which performs encoding with code rate R₂. This generates a secondencoded packet 107 which is modulated with user two modulator 110 withmodulation type M₂. The output of user two modulator 110 is a second setof modulation symbols 111 containing symbols P₁, . . . , P_(N). Theencoding and modulation performed for the first data stream 100 may bedifferent than that performed for the second data stream 102.

The first set of modulation symbols 109 is demultiplexed withdemultiplexer 112. This splits the first set of modulation symbols intoa first subset 116 designated as α₁₁. In the illustrated example, thisconsists of symbols S₁, . . . , S_(N/2−1). The demultiplexer 112produces a second subset 118 designated as α₁₂ containing symbolsS_(N/2), . . . , S_(N). More generally, any division of the symbols ofthe first set of modulation symbols 109 between two demultiplexedstreams 116,118 can be performed.

Similarly, the second set of modulation symbols 111 is demultiplexedwith demultiplexer 114 to generate two outputs 120,122. The first output120 is designed at α₂₁ and contains symbols P₁, . . . , P_(N/2−1). Thesecond output 122 is designated as α₂₂ and contains symbols P_(N/2), . .. , P_(N). α₁₁, α₁₂, α₂₁, α₂₂ are analogous to the like identifiedelements of FIG. 3. Next, part of the modulation symbols of the firstset 109, namely α₁₁, in combination with part of the modulation symbolsof the second set of modulation symbols 111, namely α₂₁ are input to aWalsh spreading block 130 for a first transmit antenna 134. Similarly,the rest of the modulation symbols, namely those of α₁₂ and α₂₂ areinput to a second Walsh spreading block 132 connected to the secondtransmit antenna 136.

For this example, it is assumed that the Walsh space W available forthis contains four Walsh codes w_(a), w_(b), w_(c), w_(d), and that twoWalsh sub-spaces are defined according to w₁={w_(a),w_(b)} andw₂={w_(c),w_(d)}. This is summarized generally at 140 of FIG. 4A. Moregenerally, the system will have available some number of Walsh codes andtypically a sub-set of these Walsh codes would be made available fortransmission of these packets. If there are a number N_(tot) of Walshcodes of which N_(data) are used to transmit packets, then a Walshspreading gain of N_(tot) divided by N_(data) is realized by the Walshspreading functions.

Referring back to FIG. 4A again, both Walsh spreading functions 130,132operate on the same Walsh space. Half of the Walsh space, namely w₁ isapplied to the data of one encoder packet while the other half of theWalsh space, namely w₂ is applied to the data of the other encode packetby each Walsh spreading function. This was summarized in FIG. 3 byindicating that the first Walsh spreading function 130 (64 of FIG. 3) isequal to w₁α₁₁+w₂α₂₁ while the second Walsh spreader 132 (spreader 66 ofFIG. 3) generates w₂α₁₂+w₁α₂₂. This is however a short hand way ofshowing how Walsh codes are applied to symbols. A detailed breakdown ofthe content of the signal transmitted on the first antenna 134 issummarized in the box 142 of FIG. 4. During the first transmit period,T₁, the output on the first antenna 134 isw_(a)S₁+w_(b)S₂+w_(c)P₁+w_(d)P₂. Similar signals are transmitted foreach of the transmit periods. It will be necessary to transmit N/4transmit periods to transmit the entire set of modulation symbols outputby the first Walsh spreading function 130. A similar breakdown of thecontent of the signal transmitted on antenna 136 is indicated at 144.

It can be seen that during a given instant of transmission, the signaltransmitted on the first antenna includes content generated from thefirst set of modulation symbols 109 which in turn was generated from thefirst encoded packet 105 and the first input data stream 100, and alsocontains content derived from a second set of modulation symbols 111which was generated from the second encoded packet 107 which wasgenerated from the second data stream 102. The same is true for thesignal transmit on the second antenna 136.

Also shown in FIG. 4A is a first adaptive control and modulation controlfunction 145 which would be featured in embodiments having adaptivecoding and modulation. This function 145 decides on the coding rate R₁and the modulation type M₁ for the first user. The decision is made as afunction of a user one reverse link control channel 146. Also shown is asecond adaptive coding and modulation control function 149 which doesthe same for the second user based on user two reverse link controlchannel 147. In one embodiment of the invention, the reverse linkcontrol channels 146,147 are used by the receivers to convey average C/Iinformation to the transmitter. The control aspect is described infurther detail below. Also shown is a user one forward link controlchannel 148 over which any control information required for user onereceiver to decode/receive the transmit information is sent. Similarly auser two forward link control channel 151 is also shown. It is notedthat the forward link control channels and the reverse link controlchannels can be transmitted using any appropriate medium. For example,the forward link control channel might be transmitted on one or moreadditional Walsh codes.

It is to be understood that FIG. 4A is a very specific implementation. Areal implementation might include additional blocks not shown.Furthermore, the number of Walsh codes used, and the manner in which theWalsh codes are divided, and the manner in which the symbols aredemultiplexed between the two antennas is specific to this example only.

As discussed above, preferably the rate and modulation type used in thetransmitter is adaptively controlled as a function of conditions at thereceiver. For this purpose, the reverse link control channels 146,147are provided. One such channel is used by each receiver to indicate tothe base station the channel quality measurements of the best servingsector. The channel quality can be measured in terms ofcarrier-to-interference ratio (CIR) and may for example be estimatedfrom the received forward-link pilot signal from each sector in theactive set. It is noted that for a conventional MIMO system, two suchchannels are required for each receiver to indicate CIRs for twotransmit antennas due to the independent rate control. For the MATsystem, however, only one channel per receiver for channel indication isrequired because only the averaged CIR is to the reported base station.The reported CIR can be averaged over all transmit antennas, given by

$\overset{\_}{CIR} = {\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{CIR}_{m}}}$where CIR_(m) denotes the received CIR from the m-th transmit antenna.In order to determine CIR_(m) the following parameters according to [4]can be defined. Let M and N denote the number of transmit and receiveantennas respectively. Let H denote the N×M channel matrix, h_(m) denotethe m-th column of the channel matrix H and H_(m)=[h₁h₂ . . .h_(m−1)h_(m+1) . . . h_(M)], i.e., the channel matrix with the m-thcolumn removed. By employing the minimum-mean square error (MMSE)receiver, the received CIR are given by

${CIR}_{m} = {h_{m}^{H} \cdot \left( {{H_{m}H_{m}^{H}} + {\frac{M}{P_{m}} \cdot I_{N}}} \right)^{- 1} \cdot h_{m}}$where the superscript H denotes conjugate and transpose, I_(N) is theidentity N×N matrix and P_(m) is the transmit power from the m-thtransmit antenna.

According to the reported CIR, the base station determines anappropriate payload. This might include adjusting transmission rate,encoding type/rate, and/or modulation type for example. Many forms ofadaptive encoding and modulation are known and can be applied here. Theinformation relative to the payload and the transmission is transmittedthrough the respective forward link control channel to each user. It isalso to be understood that other measures of channel quality canalternatively be employed within the scope of the invention.

The single user version of FIG. 4A is shown in FIG. 4B. With thisimplementation, only a single encoder and modulator are required andthese are used to generate two encoded packets and two sets ofmodulation symbols. These are then demultiplexed and spread between thetwo antennas as in the two user embodiment of FIG. 4A. In this case,there is only a single reverse link control channel 146 and a singleforward link control channel 148.

If it is assumed the same user occupies the same packet data channels,it is straightforward to understand that the PARC requires two forwardlink control channels while the proposed MAT requires only one channeldue to the same payload and transmission rate on different transmitantennas.

The comparison of signaling channels on both links between PARC and MATis listed in Table 1.

TABLE 1 Comparison of signaling channels between PARC and MAT. PARC MATReverse-Link 2 reverse link 1 reverse link control channels controlchannel per user per user Forward-Link 2 forward link 1 or 2 forwardcontrol channels link control channels

To investigate the performances in terms of user throughput, residualFER and PDCCH (packet data control channel) FER, (the PDDCH is anexample of a forward link control channel), a system level simulationassociated with full buffer FTP traffic model was employed. Thesimulation is performed to compare the following three schemes: transmitdiversity, MIMO PARC, and proposed MAT.

Simulation assumptions and methodologies are mainly based on 1xEV-DVsystem [1]. A mixed channel model as listed in Table 2 is considered inthe system level simulation.

TABLE 2 Channel Models Channel # of Speed Assignment Model fingers(km/h) Fading Probability Model A 1 3 Jakes 0.3 Model B 1 10 Jakes 0.3Model C 1 30 Jakes 0.2 Model D 1 120 Jakes 0.1 Model E 1 0, Rician 0.1fD = 1.5 Hz Factor (K = 10 dB)

With respect to both PARC and MAT simulations, the following userscheduling methods based on proportional fairness (PF) were considered.

Transmit packets belonging to different users are assigned to differenttransmit antennas (given a name diff-user in the following discussion).This provides the best performance among the schedulers since the userencoder packets do not need to occupy the same number of slots so thatmany choices relative to transmission rate and payload for the encoderpacket can be taken. In this case, two PDCCH channels are required toindicate the packet pointer.

Transmit packets must belong to the same user and occupy the same numberof slots (given a name same-user in the following discussion). Thisresults in some degradation because the determination relative topayload and transmission rate is constrained with the same number ofoccupying slots. In this case, only one PDCCH channel is required toindicate the packet pointer.

To maintain the PDCCH FER under expected level of 10⁻² in the systemlevel simulations, the margins as used in 1xEV-DV [1] for PDCCH areemployed. The specific margin values with respect to channel model andthe number of slots are listed in Table 3.

TABLE 3 Power margin f or PDCCH in 1xEV-DV. Channel Channel ChannelChannel Channel A B C D E 1 Slot 2 dB 4 dB 11 dB  16 dB  1 dB 2 1 dB 1dB 6 dB 3 dB 1 dB Slots 4 1 dB 1 dB 1 dB 1 dB 1 dB Slots

FIG. 5 and FIG. 6 show the CIR distribution for 1xEV-DV, MIMO PARC, 2×2transmit diversity, and MAT. From the figures, a comparison can be madebetween them in terms of average CIR, standard deviation, and usercoverage.

From the distribution results, it can be seen that 1xEV-DV, MIMO PARC,and MAT generate almost the same average CIR. The reason for this is asfollows. Since MIMO PARC and the MAT both employ 2×2 antennas and eachtransmits half power, the average received power combined at two receiveantennas should be same as that in 1xEV-DV. This results in the sameaverage CIR after multiple receive antennas combining. On the otherhand, because 2×2 transmit diversity combines all the paths at thereceiver, it achieves a 3 dB average CIR gain as opposed to the others.

Moreover, it is found that the MAT approach provides the same standarddeviation of received CIR as 2×2 diversity, but it is much smaller thanthat of 1xEV-DV and MIMO PARC. This is because both MAT and 2×2diversity offer an additional transmit diversity.

MAT also shows the much better user coverage than 1xEV-DV and MIMO PARC,although it is worse than 2×2 (see the CIR region between −20 and −5dB).

FIG. 7 shows the aggregate user throughput as a function of geometry forfull queue FTP with mixed channel model. It can be found that the PARCand MAT achieve a significant user throughput gain as opposed to 2×2transmit diversity, particularly in the region of large geometry. (Bothschemes show almost the same performance.) The detailed comparisonbetween the three schemes is listed in Table 4.

TABLE 4 Comparison between PARC, 2 × 2 diversity and proposed MAT.Sector Throughput Residual (kbps) FER PDCCH FER PARC Same 1716.78.59E−03 2.27E−02 User Diff 1986.1 9.63E−03 1.71E−02 User 2 × 2 — 1412.92.59E−04 3.22E−03 Diversity MAT Same 1885.7 1.27E−03 7.84E−03 User Diff1921.7 1.20E−03 7.44E−03 User

FIG. 8 shows the residual FER as a function of geometry for full queueFTP with mixed channel model. It can be found that the PARC provides thehighest residual FER while 2×2 transmit diversity provides the lowestresidual FER. The difference is significantly large particularly forhigh velocity users. Since MAT experiences the same CIR from differenttransmit antennas due to mix-antenna transmission, it may achieve theadditional transmit diversity gain. This results in an extremely lowresidual FER as compared to PARC. The detailed comparison between thethree schemes is listed in Table 4.

FIG. 9 shows the packet data control channel (PDCCH) FER as a functionof geometry for full queue FTP with mixed channel model. It is quitesimilar to the residual FER we discussed above, that the PARCexperiences the higher PDCCH FER while 2×2 transmit diversity and MATexperience the lower PDCCH FER. This is because the CIR variationsexperienced by the 2×2 transmit diversity and MAT is much smaller thanthat by PARC. The detailed comparison between the three schemes islisted in Table 4.

From the results as listed in Table 4, one can observe that the PARCexperiences higher PDCCH FER (about 2%) while the proposed MATexperiences lower PDCCH FER (about 0.7%). To make a fair comparison, thefollowing changes for the proposed MAT are considered:

a) Reduce the power margin for the proposed MAT as listed in Table 5instead of Table 3:

TABLE 5 Power margin of PDCCH for MAT. Channel Channel Channel ChannelChannel A B C D E 1 Slot   1 dB   2 dB 4 dB 8 dB 0.5 dB 2 0.75 dB   1 dB2 dB 3 dB 0.5 dB Slots 4  0.5 dB 0.5 dB 1 dB 1 dB 0.5 dB Slots

b) Employ the STS for PDCCH channel in case of the same-user served bytwo transmit antennas;

c) Reduce the number of maximum retransmissions from three to two.

By considering above changes for MAT, the simulations were performedagain. The results are shown in Table 6 for comparison.

TABLE 6 Comparison between PARC, 2 × 2 diversity and NAT. SectorThroughput Residual (kbps) FER PDCCH FER PARC Same User 1716.7 8.59E−032.27E−02 Diff User 1986.1 9.63E−03 1.71E−02 2 × 2 — 1412.9 2.59E−043.22E−03 Diversity MAT Same User 1914.5 1.38E−03 1.15E−02 (3 Diff User1930.9 l.16E−03 1.11E−02 Retrans.) MAT Same User 2002.3 7.11E−031.17E−02 (2 Diff User 1990.2 7.72E−03 1.14E−02 Retrans.)

From this table, it can be observed that with three maximumretransmissions the MAT achieves about 36% sector throughput gain asopposed to 2×2 transmit diversity and 12% to PARC in case of thesame-user on different transmit antennas. In terms of the residual FER,MAT outperforms PARC by a factor of more than eight. To maintain thesame residual FER as PARC, moreover, MAT only requires a maximum of tworetransmissions.

From Table 4 and Table 6, MAT compared to schemes of 2×2 diversity andPARC can be summarized as follows:

a) MAT achieves almost the same sector throughput as PARC, but with anextremely low residual FER (by a factor of ten);

b) MAT experiences almost the same residual FER as 2×2 transmitdiversity, but provides a significantly high sector throughput (morethan 30%);

c) MAT not only achieves the Walsh code reuse gain as PARC, but alsoprovides the transmit antenna diversity gain;

d) MAT requires less signaling channel (each signaling channel on eachlink) than PARC (two signaling channels on each link for 2×2 MIMO);

e) MAT may reduce the number of maximum retransmissions (from three totwo).

In preferred embodiments, different antennas use distinct sets of Walshcodes. More generally, the same set of codes could be used for the sameuser on different antennas as long as the eigenvalues corresponding totwo antennas are different, i.e. so long as the antennas aresubstantially uncorrelated.

REFERENCES

-   [1] 1xEV-DV Evaluation Methodology—Addendum (V6), Jul. 25, 2001.-   [2] Updated Joint Physical Layer Proposal for 1xEV-DV, Jun. 11,    2001.-   [3] Lucent Technologies, MIMO Architecture Proposal for the F-PDCH,    TSG-C, WG3, Seattle, Wash., C30-20020204-051, Feb. 5, 2002.-   [4] S. M. Alamouti, “A simple transmit diversity technique for    wireless communications”, IEEE JSAC, vol. 16, pp. 1451-58, October    1998.-   [5] S. T. Chung, A. Lozano, and H. Huang, “Approaching eigenmode    BLAST channel capacity using V-BLAST with rate and power feedback”,    VTC 2001, Atlantic City, N.J., October 2001.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practised otherwise than as specifically described herein.

1. A method of receiving a set of M signals using a receiver having Mantennas, where M>=2, the method comprising: for each signal of the setof M signals: Spreading each one of M subsets of modulation symbols,each of the M subsets of modulation symbols comprising at least twomodulation symbols of the signal, by using a respective one of Mdisjoint orthogonal codes to generate a set of M corresponding spreadsignal components, each of the M corresponding spread signal componentsof the set of M corresponding spread signal components including adifferent one of the disjoint orthogonal codes; and for each antenna:Generating a respective combined signal having one spread signalcomponent of each signal by including the different one of the Mdisjoint orthogonal codes in each of the M corresponding spread signalcomponents of the respective combined signal; and receiving therespective combined signal through the antenna.
 2. A method according toclaim 1 further comprising: generating an average signal quality metricfrom the signals and transmitting the average signal quality metric backto the transmitter.
 3. A receiver for receiving a set of M signals usingM antennas, where M>=2, the receiver comprising: for each signal of theset of m signals: A spreading function for spreading each one of Msubsets of modulation symbols, each of the M subsets of modulationsymbols comprising at least two modulation symbols of the respectivesignal, by using a respective one of M disjoint orthogonal codes togenerate a set of M corresponding spread signal components, each of theM corresponding spread signal components of the set of M correspondingspread signal components including a different one of the M disjointorthogonal codes; Means for generating M combined signals, each of the Mcombined signals including one spread signal component of each signal,by including the different one of the M disjoint orthogonal codes ineach of the M corresponding spread signal components of the M combinedsignals; and a respective antenna for receiving each of the M combinedsignals.
 4. A receiver according to claim 3 further adapted to generatean average signal quality metric from the signals and transmit theaverage signal quality metric back to the transmitter.
 5. A method oftransmitting a set of M signals using a transmitter having M antennas,where M>—2, the method comprising: for each signal of the set of Msignals: spreading each one of M subsets of modulation symbols, each ofthe M subsets of modulation symbols comprising at least two modulationsymbols of the signal, by using a respective one of M disjointorthogonal codes to generate a set of M corresponding spread signalcomponents, each of the M corresponding spread signal components of theset of M corresponding spread signal components including a differentone of the M disjoint orthogonal codes; and for each antenna: generatinga respective combined signal having one spread signal component of eachsignal by including the different one of the M disjoint orthogonal codesin each of the M corresponding spread signal components of therespective combined signal; and transmitting the respective combinedsignal through the antenna.
 6. A method according to claim 5 wherein theM disjoint orthogonal codes are Walsh codes.
 7. A method according toclaim 5 wherein the modulation symbols of each signal are generated by:encoding a raw information stream to generate corresponding encodedpackets; and modulating the corresponding encoded packets to generatethe modulation symbols of the signal.
 8. A method according to claim 7further comprising, for each signal: receiving respective channelquality information for the signal over a reverse link control channel,the respective channel quality information being generated by a receiverof the signal; and adaptively controlling the steps of encoding the rawinformation stream and modulating the encoded packets as a function ofthe received respective channel quality information.
 9. A methodaccording to claim 8 wherein the respective channel quality informationfor the signal comprises an average carrier to interference ratioexperienced across multiple antennas at a receiver of the signal.
 10. Amethod according to claim 5 wherein the M signals of the set of Msignals belong to at least one user, the method further comprising: foreach user, transmitting a respective forward link control channelidentifying one or more of a number of slots, a transmission rate, acode rate/type and a modulation type.
 11. A transmitter for transmittinga set of M signals using M antennas, where M>=2, the transmittercomprising: for each signal of the set of M signals: a spreadingfunction for spreading each one of M subsets of modulation symbols, eachof the M subsets of modulation symbols comprising at least twomodulation symbols of the respective signal, by using a respective oneof M disjoint orthogonal codes to generate a set of M correspondingspread signal components, each of the M corresponding spread signalcomponents of the set of M corresponding spread signal componentsincluding a different one of the M disjoint orthogonal codes; means forgenerating M combined signals, each of the M combined signals includingone spread signal component of each signal, by including the differentone of the M disjoint orthogonal codes in each of the M correspondingspread signal components of the M combined signals; and a respectiveantenna for transmitting each of the M combined signals.
 12. Atransmitter according to claim 11 wherein the M disjoint orthogonalcodes are Walsh codes.
 13. A transmitter according to claim 11 furthercomprising, for each signal: an encoder for encoding a raw informationstream to generate corresponding encoded packets of the signal; amodulator for modulating the corresponding encoded packets to generatethe modulation symbols of the signal.
 14. A transmitter according toclaim 13 further comprising, for each signal: an input for receivingrespective channel quality information for the signal over a reverselink control channel, the respective channel quality information beinggenerated by a receiver of the signal; and an adaptive controller foradaptively controlling the encoder and the modulator as a function ofthe received respective channel quality information.
 15. A transmitteraccording to claim 11 wherein the M combined signals belong to at leastone user, the transmitter being further configured to transmit arespective forward link control channel for each user, each respectiveforward link control channel identifying one or more of a number ofslots, a transmission rate, a code rate/type and a modulation type.